Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants. Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems. When used in compression or absorption systems, these fluids provide refrigeration by undergoing a phase change process in the evaporator.
As the name implies, secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes.for example in large air conditioning systems. Antifreezes or brines are used when refrigeration is required at sub-zero temperatures. Unlike primary refrigerants, the secondary refrigerants do not undergo phase change as they transport energy from one location to other. An important property of a secondary refrigerant is its freezing point. Generally, the freezing point of a brine will be lower than the freezing point of its constituents
Designation of refrigerants:
i) Fully saturated, halogenated compounds: These refrigerants are derivatives of alkanes (CnH2n+2) such as methane (CH4), ethane (C2H6). These refrigerants are designated by R XYZ, where:
X+1 indicates the number of Carbon (C) atoms
Y-1 indicates number of Hydrogen (H) atoms, and
Z indicates number of Fluorine (F) atoms
The balance indicates the number of Chlorine atoms. Only 2 digits indicates that the value of X is zero.
Ex: R 22
X = 0 ⇒ No. of Carbon atoms = 0+1 = 1 ⇒ derivative of methane (CH4)
Y = 2 ⇒ No. of Hydrogen atoms = 2-1 = 1
Z = 2 ⇒ No. of Fluorine atoms = 2
The balance = 4 – no. of (H+F) atoms = 4-1-2 = 1 ⇒ No. of Chlorine atoms = 1
∴The chemical formula of R 22 = CHClF2
Similarly it can be shown that the chemical formula of:
R12 = CCl2F2
R134a = C2H2F4 (derivative of ethane)
(letter a stands for isomer, e.g. molecules having same chemical composition but different atomic arrangement, e.g. R134 and R134a)
ii) Inorganic refrigerants: These are designated by number 7 followed by the molecular weight of the refrigerant (rounded-off).
Ex.: Ammonia: Molecular weight is 17, ∴ the designation is R 717
Carbon dioxide: Molecular weight is 44, ∴ the designation is R 744
Water: Molecular weight is 18, ∴ the designation is R 718
iii) Mixtures: Azeotropic mixtures are designated by 500 series, where as zeotropic refrigerants (e.g. non-azeotropic mixtures) are designated by 400 series.
Azeotropic mixtures:
R 500: Mixture of R 12 (73.8 %) and R 152a (26.2%)
R 502: Mixture of R 22 (48.8 %) and R 115 (51.2%)
R503: Mixture of R 23 (40.1 %) and R 13 (59.9%)
R507A: Mixture of R 125 (50%) and R 143a (50%)
Zeotropic mixtures:
R404A : Mixture of R 125 (44%), R 143a (52%) and R 134a (4%)
R407A : Mixture of R 32 (20%), R 125 (40%) and R 134a (40%)
R407B : Mixture of R 32 (10%), R 125 (70%) and R 134a (20%)
R410A : Mixture of R 32 (50%) and R 125 (50%)
iv) Hydrocarbons:
Propane (C3H8) : R 290
n-butane (C4H10) : R 600
iso-butane (C4H10) : R 600a
Unsaturated Hydrocarbons: R1150 (C2H4)
R1270 (C3H6)
Refrigerant selection criteria:
Selection of refrigerant for a particular application is based on the following requirements:
i. Thermodynamic and thermo-physical properties
ii. Environmental and safety properties, and
iii. Economics
Thermodynamic and thermo-physical properties:
The requirements are:
a) Suction pressure: At a given evaporator temperature, the saturation pressure should be above atmospheric for prevention of air or moisture ingress into the system and ease of leak detection. Higher suction pressure is better as it leads to smaller compressor displacement
b) Discharge pressure: At a given condenser temperature, the discharge pressure should be as small as possible to allow light-weight construction of compressor, condenser etc.
c) Pressure ratio: Should be as small as possible for high volumetric efficiency and low power consumption
d) Latent heat of vaporization: Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small
e) Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be small
f) Liquid specific heat: Should be small so that degree of subcooling will be large leading to smaller amount of flash gas at evaporator inlet
g) Vapour specific heat: Should be large so that the degree of superheating will be small
h) Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficients
i) Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops
Environmental and safety properties:
a) Ozone Depletion Potential (ODP): According to the Montreal protocol, the ODP of refrigerants should be zero, i.e., they should be non-ozone depleting substances. Refrigerants having non-zero ODP have either already been phased-out (e.g. R 11, R 12) or will be phased-out in near-future(e.g. R22). Since ODP depends mainly on the presence of chlorine or bromine in the molecules, refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be used under the new regulations
b) Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming. Refrigerants with zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future.
c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming. Naturally, refrigerants with as a low a value of TEWI are preferable from global warming point of view.
Other important properties are:
f) Chemical stability: The refrigerants should be chemically stable as long as they are inside the refrigeration system.
g) Compatibility with common materials of construction (both metals and non-metals)
h) Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. ammonia). Refrigerants that are completely miscible with oils are easier to handle (e.g. R12). However, for refrigerants with limited solubility (e.g. R 22) special precautions should be taken while designing the system to ensure oil return to the compressor
i) Dilelectric strength: This is an important property for systems using hermetic compressors. For these systems the refrigerants should have as high a dielectric strength as possible
j) Ease of leak detection: In the event of leakage of refrigerant from the system, it should be easy to detect the leaks.
QUIZZZZZ
1. Which of the following statements are TRUE?
a) A primary refrigerant does not undergo phase change in a refrigeration cycle
b) A secondary refrigerant does not undergo phase change in a refrigeration cycle
c) The freezing point of a brine is generally lower than the freezing point of its constituents
d) The freezing point of a brine is generally higher than the freezing point of its constituents
2. Which of the following statements are TRUE?
a) The suction pressure of a refrigerant should be as high as possible
b) The suction pressure of a refrigerant should be as low as possible
c) The discharge pressure of a refrigerant should be as high as possible
d) The discharge pressure of a refrigerant should be as low as possible
3. The chemical formula of refrigerant R11 is:
a) CCl3F
b) CClF3
c) CClHF
d)CHF
4. The chemical formula of R141 is:
a) C2H3ClF3
b) C2H2Cl3F
c) C2H3Cl2F
d) C2H2ClF3
5. Which of the following refrigerants replace R12 in domestic refrigerators?
a) R22
b) R11
c) R134a
d) R141b
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